Catabolism is a key determinant of the actual steady-state level of a particular metabolite. For example, levels of oligosaccharides and complex carbohydrates such as starch in plants are determined by both enzymes involved in their synthesis as well as enzymes involved in their degradation. Among the enzynes involved in carbohydrate catabolism is alpha-glucosidase. An exo-carbohydrase, the enzyme hydrolyzes the terminal, non-reducing 1,4-linked D-glucose residues in malto-oligosaccharides and starch releasing D-glucose. Different isoforms of alpha-glucosidase that vary in substrate specificities and pH optima have been characterized in plants. In spinach, isoforms I and II use starch and a range of malto-oligosaccharides as substrates whereas isoforms III and IV only use malto-oligosaccharides (Sugimoto et al. (1997) Plant Mol Biol 33:765-768). In mammalian systems, glucosidase I and glucosidase II are known to be sequentially involved in the trimming of glucose residues from N-linked glycans on newly synthesized glycoproteins in the endoplasmic reticulum (ER) (Kalz-Fuller et al. (1995) Eur J Biochem 231:344-351; Trombetta et al. (1996) J Biol Chem 271:27509-27516). Alpha-glucosidase II is composed of two subunits, alpha and beta, with the alpha subunit harboring the catalytic activity, while the smaller subunit which contains a putative ER retention signal (HDEL) at the C terminus, may be involved in localizing the enzyme to the ER (Trombetta et al. (1996) J Biol Chem 271:27509-27516).
Plant glucosidase genes have been isolated from various species, including spinach (Sugimoto et al. (1997) Plant Mol Biol 33:765-768), potato (Taylor et al. (1998) Plant J 13:419-425), and barley (Tibbot and Skadsen (1996) Plant Mol Biol 30:229-241; U.S. Pat. No. 5,763,252). Expectedly, high gene expression is observed during germination stages, when starch reserves are broken down to supply the energy needed for growth (Tibbot and Skadsen (1996) Plant Mol Biol 30:229-241; Taylor et al. (1998) Plant J 13:419-425). There is a great deal of interest in identifying the genes that encode alpha-glucosidases in plants. These genes may be used in plants to control carbohydrate levels in plants and seeds or to manipulate glycoprotein processing. Accordingly, the availability of nucleic acid sequences encoding all or a portion of these alpha-glucosidase II enzymes would facilitate studies to better understand plant metabolism, and provide genetic and biochemical tools for the manipulation of the starch/sucrose content in plants and seeds, and mass-production of the enzyme for post-harvest starch modification.